1. Technical Field of the Invention
The present invention relates generally to semiconductor devices, and more particularly to a concentrated photovoltaic (CPV) receiver cell which includes through silicon via (TSV) or through wafer via (TWV) structures etched therethrough to create a connector at the base of the cell, thus providing the cell with greater surface area for solar input.
2. Description of the Related Art
Photovoltaic cells are a well known means for producing electrical current from electromagnetic radiation. Traditional photovoltaic cells comprise junction diodes fabricated from appropriately doped semiconductor materials. Such devices are typically fabricated as thin, flat wafers with the junction formed parallel to and near one of the flat surfaces. Photovoltaic cells are intended to be illuminated through their so-called “front” surface. Electromagnetic radiation absorbed by the semiconductor produces electron-hole pairs in the semiconductor. These electron-hole pairs may be separated by the electric field of the junction, thereby producing a photocurrent. Currently known photovoltaic cells typically have a generally quadrangular (e.g., square) configuration defining four peripheral side edges, and include a pair of bus bars which are disposed on the top or front surface and extend along respective ones of an opposed pair of the side edges. The bus bars are used to facilitate the electrical connection of the photovoltaic cell to another structure, as described in more detail below.
There is currently known in the electrical arts semiconductor devices known as CPV receiver die packages or modules. Currently known CPV modules typically comprise a ceramic substrate having a conductive pattern disposed on one side or face thereof. Attached to the substrate and electrically connected to the conductive pattern are electrical components, including a pair of preformed wire connectors and a packaged diode. Also attached to the substrate and electrically connected to the conductive pattern thereof is a CPV receiver cell or die. The electrical connection between the receiver die and the conductive pattern is often facilitated by a pair of punched thin metal foil or braided ribbon/mesh connectors which extend along and are welded or soldered to respective ones of opposed sides of the receiver die, which typically has a quadrangular or square configuration as indicated above. More particularly, the pair of punched thin metal foils or braided ribbon/mesh connectors are welded or soldered to respective ones of the bus bars on the top or front surface of the receiver die. In certain existing CPV modules, the electrical connection of the receiver die to the conductive pattern is facilitated by the use of multiple wires bonded to the bus bars on the front surface of the receiver die and the bond pads of the conductive pattern of the substrate, the wires being used as an alternative to the aforementioned braided ribbon or mesh interconnects. The CPV module may further include a light concentration means which is adapted to concentrate solar radiation onto the front surface of the receiver die.
Current CPV receiver die packages or modules typically generate up to ten amps of electrical current. In order to carry such high current, the above-described ribbons made of metal foil or braided wire mesh, or the above-described multiple bond wire bonds are used to form the interconnection between the bus bars on the front surface of the receiver die and the bond pads of the conductive pattern on the substrate. However, the use of the ribbon/mesh type interconnects or, alternatively, the bond wires give rise to certain deficiencies in currently known CPV modules which detract from their overall utility. More particularly, the ribbon/mesh type interconnects do not have good shape control for automatic pick up, and require the use of specialized welding equipment for the fabrication of the CPV module using the same. Stated another way, it is often difficult to control the shape of the ribbon/mesh type interconnects for automatic pick up and placement, with the fabrication process being mostly done through the use of special welding equipment or manual soldering which is more labor intensive and thus more costly. When wires are used as an alternative to the ribbon/mesh type interconnects, such wires require encapsulation protection for long-term reliability of the CPV module including the same. In addition, in those CPV modules including bond wires, problems may arise in relation to current crowding if too few wires are used. As indicated above, the wires also require encapsulation, over-molding, or other protection from the environment. Moreover, the use of the soldered or welded ribbon/mesh interconnects or bond wires create concerns regarding the electrical current carrying capability of the CPV module including the same.
As indicated above, the inclusion of the bus bars on the front surface of the receiver cell or die necessitates that the above-described ribbons made of metal, foil or braided wire mesh, or bond wires, be used to facilitate the electrical connection of the receiver die to the bond pads of the conductive pattern on the underlying substrate. In addition to the use of the ribbon/mesh type interconnects or wires giving rise to the deficiencies highlighted above, further penalties in the potential performance of the receiver die are directly attributable to the inclusion of the bus bars on the top or front surface of the receiver die. More particularly, in currently known concentrated photovoltaic receiver cells or dies, a certain percentage of the total area of the front surface of the receiver die is covered by the bus bars. That percentage of the total die area of the front surface covered by the bus bars is typically in the range of from about 8% to about 10%. As will be recognized, the active area of the front surface as a percentage of the overall die area thereof is thus reduced by an amount equal to the percentage of the area covered by the bus bars. In this regard, the active area as a percentage of the overall die area of the front surface could be substantially increased and the receiver die thus made more efficient if the bus bars on the front surface were effectively eliminated from the receiver die.
The present invention addresses these and other shortcomings of prior art CPV receiver dies by providing a concentrated photovoltaic receiver cell or die wherein an etching process is used to create through wafer vias or TWV's through the receiver die. The TWV's in turn are used to create connectors at the back or bottom surface of the receiver die, thus eliminating the need for the bus bars on the front surface thereof. The movement of the connectors (e.g., bus bars) to the bottom surface of the receiver die provides the potential for a greater active surface area on the front surface for solar input. The movement of the connectors to the bottom surface of the receiver die also effectively eliminates the need for top side connectors such as the aforementioned ribbon/mesh type interconnects or wires, thus allowing for easier top side connection for prisms or other optical input devices. Thus, the receiver die constructed in accordance with the present invention provides an increased top, active surface area ratio, which provides higher wafer utilization and higher power per area efficiency. Additionally, as indicated above, the elimination of the external wires or mesh/ribbon interconnects provides a reliability enhancements and/or manufacturing economies. These and other features of the present invention will be described in more detail below.